Conditions for optimal net synthesis of secretory granules in rat parotid slices will be sought. The granules normally contain exportable proteins and large amounts of calcium. Net synthesis under controlled conditions in vitro will permit the study of several important questions: 1) Is calcium required for the synthesis of exportable protein? 2) Is calcium required for the transport of exportable protein? 3) Is calcium required for the assembly of the secretory granules? 4) Are granules formed which do not contain calcium and are these functional in secretion? 5) Can the relative rates of synthesis of the different secretory proteins be modulated by changing the amino acid composition of the medium?